virt-libvirt: Pylint fixes for vm_xml

Mainly minor items that should have no effect on tests.  Some
more risky changes were made to set_agent_channel() and
get_iface_dev() methods.  Those changes were made because
it made line-wrapping easier (lines were too long) and to
make parameter passing more clear (redundant ** magic).

Signed-off-by: Chris Evich <cevich@redhat.com>
virt-libvirt: Unittest causing failures in others

Our check_patch and run_unittests scripts both run all unittest files IN
PARALLEL, not one at a time. This leads to some really interesting and
difficult to debug problems, like one test deleting/modifying temp files
or state for another test.  Problems here are:

1) The VIRSH_EXEC logic isn't working right, and because the virsh
module is sharing memory, changing virsh.virsh_exec in your test is
interfering with other tests.

2) Many tests are calling self.sp.method() which execute real virsh
commands. This is fine, except that they don't all work w/o root access
and therefore are dependant on external state.

Signed-off-by: Chris Evich <cevich@redhat.com>